Function is_terminated 

pub fn is_terminated(subscription_state: &[u8]) -> bool

Whether a Subscription-State says the subscription is over (RFC 6665 §4.1.3).

Asks the event framework rather than reading the header again. The implicit subscription a REFER creates is a subscription — S-13 made that a thing sipx has a general answer for — and two parsers for one header eventually disagree about whether a transfer has finished.